5.3 Ground-Water Flow in the Cedar Hills Aquifer

The ground-water flow system in the Cedar Hills aquifer can be
subdivided areally into two parts based on the configuration of the
potentiometric surface. Figure 19 shows the potentiometric surface
of the Cedar Hills aquifer in the study area. Data used to prepare
this map came from surveys run during the 1970's by the Kansas
Department of Health and Environment and were supplied by Bill
Bryson, KCC. Additional data were developed from measurements
made during the course of this study.

Figure 19. Potentiometric surface of the Cedar Hills aquifer.

Pre-Cretaceous erosion has removed younger Permian-aged strata
east of a north-northwest trending line that extends through
western Ellis, Rooks, and Rush counties. Eastward of this line, the
Cedar Hills underlies and is believed to be hydraulically
interconnected with the Great Plains aquifer. Interconnection
between the two aquifer systems is suggested by fundamental
changes in the configuration of the potentio-metric surface
eastward of this line (Figure 19). The direction of ground-water
flow in the lower part of the Great Plains aquifer is distinctly
subparallel with the eastward flow direction indicated for the Cedar
Hills aquifer in this area.

Westward of the Cedar Hills subcrop, the potentiometric surface has
been affected by the injection of oil-field brines and shows the
effects of fluid pressure build-up in southern Trego, northwestern
Rush, and western Graham counties. These areas of pressure
build-up have resulted from coalescing areas of higher fluid
pressure in the aquifer where injection is taking place and appear as
a series of isolated highs in the potentiometric surface (Figure 19).

The potentiometric surface of the Cedar Hills aquifer is generally
higher than the potentiometric surfaces in the upper and lower Great
Plains aquifer over most of the study area. Hydraulic head
differences on the order of 50 feet or more are common. However,
only limited data on the these aquifers are available in Rooks and
eastern Graham counties.

Recharge to the Cedar Hills aquifer is from two sources: underflow
from areas southwest of the study area and injection of oil-field
brines through disposal wells. Underflow to the Cedar Hills aquifer
has not been estimated from due to the irregularity of the
potentiometric surface. Recharge to the Cedar Hills aquifer from
injection wells is estimated to have been approximately 6.9 ft3/sec.
(5035 acre-feet per year) during the 1975-1983 period. Figure 20
shows the distribution of disposal wells in Russell, Ellis, Trego,
Graham, Rooks, Osborne and Barton counties.

Discharge from the Cedar Hills is believed to be to the overlying
lower portions of the Great Plains aquifer east of R19W in the study
area. Discharge from the Cedar Hills aquifer has not been estimated
due to lack of information on the vertical component of hydraulic
conductivity in the Great Plains and the Cedar Hills aquifers.

Static fluid level and injection pressure data gathered by the KCC
suggest, at least theoretically, that hydraulic fracturing of the
Cedar Hills aquifer may be taking place during injection at some of
the disposal sites. Shown in Figure 21 are the results of a survey of
shallow injection wells in Rush County conducted by KCC to
determine the variation of fluid pressure in the vicinity of the well
during injection. This was done by measuring static fluid levels and
monitoring fluid pressures, during and after injection. Plotted on
the figure are two lines showing the increase hydrostatic fluid pressure and lithostatic pressure with depth.

Also shown is a third line showing an increase in fluid pressure with
depth of 0.64 psi/ft. Hubbert and Willis (1972) have shown that
theoretically the minimum injection pressure-depth ratio required
to initiate hydraulic fracturing (normal faulting in tectonically
relaxed areas) in a well is approximately 0.64 psi/ft of depth for a
homogeneous and isotropic porous media. This assumes an effective
overburden pressure of 1.00 psi/ft which is typical for most
sedimentary rocks. The total injection pressure is calculated as the
sum of the fluid pressure in the formation and the applied injection
pressure at the surface assuming no losses due to friction and fluid
compression. Figure 21 shows that two of the injection wells
surveyed have clearly exceeded the theoretical minimum injection
pressure required for hydraulic fracturing. This theoretical
minimum injection pressure/depth ratio may be low for flat-lying,
lenticular formations if the rocks already contain fractures and
other discontinuities (Lorenz et al., 1986).